1. Field of the Invention
The invention relates to an improvement in olefin production from hydrocarbon cracking (pyrolysis) that reduces pressure drop, improves heat transfer, reduces energy requirements, and increases yield and selectivity. More particularly, the invention provides a modified radiant section of a hydrocarbon cracking reactor wherein the internal volume per unit length of a tubular or plug flow radiant heat-receiving reactor section increases from inlet to outlet and wherein the internal surfaces of the reactor are extended to provide improved heat transfer for enhanced reaction rates while not significantly adversely affecting pressure drop through the reactor.
2. Description of the Related Art
It is known to produce olefins by thermally decomposing ("pyrolyzing" or "cracking") hydrocarbons in a pyrolysis furnace supplied with tubes that comprise a tubular or plug-flow reactor through which feedstock flows and in which the feedstock is thermally decomposed. A pyrolysis furnace is designed to transfer heat to internal reactor tubes which are conventionally arranged in three sections: a convection section, in which the hydrocarbon feedstock is preheated; a radiant section, in which the preheated hydrocarbon feedstock is thermally decomposed to olefins, diolefins, and aromatics; and a quench section where the effluent from the radiant section is cooled.
In order to enhance selectivity and yield of a desired olefin, attention is usually focused on the radiant section of the reactor where most of the thermal cracking reactions occur. A typical radiant section of a pyrolysis reactor consists of tubes or coils arranged within the furnace so that they are subject to radiant heat from burning fuel. The hydrocarbon feedstock resides in this radiant section for only a short period of time (referred to as the radiant tube "residence time"). It is generally recognized that the residence time in the radiant section should be as short as possible to arrest other decomposing reactions so that the production of unwanted polymers and tars is minimized. Thus, for improved selectivity towards desired olefins, a short residence time regarded as optional.
Further, it is known that when the hydrocarbons undergo thermal cracking, there is a net increase in the number of moles present in the reactor. As a result, the pressure in the reactor is expected to increase due to the increase in number of moles. This pressure increase in turn acts to retard the rate of pyrolysis reaction producing desired olefins, in accordance with Le Chatelier's principle.
Generally, when feedstock enters the radiant section reactor, it is first heated up to a temperature where pyrolysis reaction rates become significant. Up to this point in the reactor, molar (and hence volume) expansion is not very significant and pressure drop is not a significant issue. Thus, the primary focus is on heat transfer to the feedstock. However, further along the reactor where the temperature is sufficiently high that rapid pyrolysis reaction takes place, pressure drop concerns arise. Typically, extended inner surface tubing used in current reactors allow rapid heat transfer to heat up incoming feedstock but the extended surfaces give rise to increased pressure drop in those sectors of the reactor where significant pyrolysis and attendant molar increase takes place.
Therefore, it would be desirable to provide a reactor wherein heat transfer is enhanced in the zone where the feedstock is initially heated up without significantly adversely affecting pressure drop and wherein the pressure does not increase, or increases only marginally, in the zones where the thermal cracking reaction predominates. It is further desirable to improve the yield of olefins, and reduce the yield of polymers and tars (i.e., improve selectivity in favor of olefins and thereby increase yield of olefins), thereby conserving energy and resources needed in the pyrolysis process.